Needle valve nozzles are used in injection moulds for supplying a flowable melt at a pre-determinable temperature at high pressure to a separable mould block (mould insert). The shut-off needles, in general pneumatically or hydraulically actuated, are designed to periodically open and close outlet openings for the melt, for example when a plastic material is to be injected in a segmental manner (cascade moulding).
Each shut-off needle is arranged in the mould-side area of the needle valve nozzle in an axially displaceable manner, and in the nozzle-side area centrically running through a melt channel (see DE 32 49 486 C3 or DE 34 03 603 A1, for example). Typically, the melt channel terminates in a nozzle mouth piece which forms on its end the outlet opening for the melt. In closed position, the lower end of the shut-off needle which has a generally cylindrical section engages into a likewise cylindrical sealing seat which is formed in the nozzle mouthpiece or in the mould insert.
In order to be able to maintain the melt at a constant temperature on its way through the tempered melt channel to the mould insert, the nozzle mouthpiece typically consists of a highly thermally conductive material. It is directly screwed into the nozzle body of the needle valve nozzle from the bottom side or, as provided in DE 197 17 381 A1, firmly secured by means of an outer screw socket made from a material of low thermal conductivity. The outer end of the screw socket engages into a corresponding cylindrical seat in the mould insert, thus centring the outlet opening with respect to the sealing seat. A narrow air gap formed between the highly thermally conductive nozzle mouthpiece and the mould insert provides the required thermal separation between the needle valve nozzle and the mould.
Due to the effects of extremely high pressures, for example far greater than 1,000 bar, a precise sealing in the closed position as well as an accurate needle guidance are essential. To this end, the nozzle mouthpiece is provided, above the outlet opening, with an infeed cone for the shut-off needle to allow the latter to centrically enter the sealing seat during closing. However, this presents the problem that the shut-off needles use to strike with their sealing edges the infeed cone in the nozzle mouthpiece. As a result, damages and in the long run leakages may occur. Precise sealing requires compliance with strict tolerance limits, in particular in the case of long nozzles and deep bores, and thus entails increased costs for manufacture and repair.
To avoid these drawbacks, the document DE 32 45 571 C2 provides a shut-off needle with a stepped lower end, such that a striking edge of increased diameter is formed ahead of the actual shut-off part of the needle. The axial length of the preferably cylindrical shut-off part, i.e. the axial distance between the front sealing edge of the shut-off part and the striking edge, is selected so as to provide an angle formed by a connection line from the sealing edge to the striking edge extending through connection points distributed on parallel diameters, which is greater than the cone angle of the feed hopper in the nozzle mouthpiece, in each case relative to the longitudinal axis of the melt channel. Thus the nozzle mouthpiece serves as a pre-centring body for the shut-off needle. In fact, whenever the shut-off needle is deflected from its central position during the closing procedure, only the striking edge will contact the infeed cone, whereas the sensitive sealing edge of the shut-off needle is introduced in a contactless manner into the nozzle mouthpiece.
To improve service life, DE 32 45 571 C2 further proposes a nozzle mouthpiece made from a wear-resistant material. Such materials, however, are poor thermal conductors and therefore impair the temperature distribution in the unheatable nozzle mouthpiece.
The same proposal is made in the document DE 41 09 122 C1, except that the wear-resistant nozzle mouthpiece is not screwed into the nozzle body but axially pressed against the latter by means of a tension ring which encloses the mouthpiece and is screwed onto the nozzle body. A further drawback is the lateral introduction of the shut-off needle into the melt channel which entails great constructional expense. Furthermore, the shut-off part of the needle is not flush with the nozzle mouthpiece and therefore produces objectionable gate marks.